If I wasn't happy in my last relationship, why am I sad we spilt up?

I broke up with my bf tonight and even though I can see logically it was the best thing, I still feel bad and I know I will have bad days after today. Why am I unable to feel relieved rather than bad that it ended? Am I just feeling confusion? I know it needed to end but Im scared of the good memories making me sad....Six years was too long maybe?